Focus detecting apparatus

ABSTRACT

A focus detecting system includes a lens device for forming an image of an object on a predetermined plane through an optical path. A light responsive device is positioned at the predetermined plane to respond to the light from the lens device for producing electrical signals representative of the instaneous intensity of the image formed thereon. A movable and a stationary apertured masks are arranged in the optical path between the lends device and the light responsive device to cyclically vary the light angle of access from the lens device to the light responsive device as the apertures in the movable mask are cyclically moved with respect to corresponding apertures in the stationary mask. The intensity distribution of the light falling on the light responsive device is modulated by the operation of the masks and varies as a function of the condition of focus of the image. A signal processing circuit responds to the electrical signals to provide an output signal indicative of condition of focus of the image.

United States Patent 1191 Stauffer Sept. 9, 1975 FOCUS DETECTINGAPPARATUS [75] Inventor: Norman L. Stauffer, Englewood,

52 u.s.c1. 250/204; 250/232; 354/25;

356/125 51 1m.c1. ..G01j 1/36 58 FieldofSearch 250/571,578, 234, 235,

Primary Examiner-Walter Stolwein Attorney, Agent, or FirmArthur H.Swanson; Lockwood D. Burton; George E. Bodenstein ABSTRACT A focusdetecting system includes a lens device for forming an image of anobject on a predetermined plane through an optical path. A lightresponsive device is positioned at the predetermined plane to respond tothe light from the lens device for producing electrical signalsrepresentative of the instaneous intensity of the image formed thereon.A movable and a stationary apertured masks are arranged in the opticalpath between the lends device and the light responsive device tocyclically vary the light angle of access from [56] References Cited thelens device to the light responsive device as the UNITED STATES PATENTSapertures in the movable mask are cyclically moved with respect tocorresponding apertures in the stationg ,Yfg f ary mask. The intensitydistribution of the light falling 3 22 J z 250/201 on the lightresponsive device is modulated by the op- 37555280 1/1971 X eration ofthe masks and varies as a function of the 36103934 10/1971 Turner": 250235 Condition of focus of the image- A Signal Processing 3.757,1249/1973 Kaestner 250/201 Circuit responds to the electrical signals toprovide an 3.776639 12/1973 Stauffer 356/126 Output signal indicative ofcondition of focus of the image.

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.. L I T i I v I T I r 1 I I I27 I29 PHOTORESPONSIVE DEVICE. PROCESSINGCIRCUIT FOCUS DETECTING APPARATUS CROSS REFERENCE TO RELATEDAPPLICATIONS Subject matter disclosed but not claimed herein isdisclosed and claimed in two copencling applications of Norman L.Stauffer, Ser. Nos. 404,318 and 404,320, both filed even date herewith.

BACKGROUND OF THE INVENTION Field of the Invention The present inventionrelates generally to automatic focussing systems; in particular, to afocus position detecting apparatus for use with cameras and the like,such as may be found in Patent Office Class 250, sub class 204.

In the past, most automatic focussing systems for use in cameras, forexample, have required two basic motions. One is the motion ofafocussing lens with respect to a plane upon which a focussed image of ascene is to be formed. The second motion is a scanning motion of a lightdetector with respect to the image at the image plane. As the lens movesfrom one extremity to another in a predetermined path of travel, a pointwill be encountered at which a major portion of a scene being formed onan image plane will be in focus. The focussed portion of the image ischaracterized by abrupt contrast changes of a focussed image. If theimage is continually scanned during the lens movement, the imagecontrast changes may be detected to provide an indication of thecondition of focus of the image through relatively complex frequencycontent analysis circuits. Some focus detecting systems have beendeveloped which obviate the need for complex electrical frequencycontent analysis circuits in order to determine the best focus positionof the objective lens. However, those systems have introduced otherdisadvantages such as being optically complex, costly, and relativelyinaccurate. Other systems rely on a comparison of signals produced bycorresponding light rcsponsive elements. Those systems, while generallysatisfactory, assume identical response characteristics of thecorresponding light responsive elements.

SUMMARY OF THE INVENTION It is, accordingly, an object of the presentinvention to provide a focus detecting apparatus which obviates thedisadvantages of prior art devices.

It is another object of the present invention to provide a focusdetecting apparatus which requires no frequency analysis in ordertodetermine the best focus position of an objective lens.

It is yet another object of the present invention to provide a focusdetecting apparatus as set forth, which is relatively simple in design.

It is a further object of the present invention to provide a focusdetecting apparatus as set forth, which is more accurate and reliablethan prior art devices.

In accomplishing these and other objects there has been provided, inaccordance with the present invention, an improved focus detectingapparatus which includcs an image forming device for forming an image ofan object through a predetermined optical path and varying the conditionof focus of that image. A masking device is arranged to cyclically maska portion of the optical path thereby repetitively varying thepositional relation of the masked portion of the optical path. A

sensing device is positioned in the optical path to respond to the lightforming the image for producing electrical signals representative of theinstantaneous intensity distribution of the image. The intensitydistribution of the light falling on the sensing device is modulated bythe masking device, and varies as a function of the condition of focusof the image. A signal processing circuit responds to the electricalsignals to provide an output signal indicative of the condition of focusof the image.

BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of the presentinvention may be had from the following detailed description, when readin connection with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of one embodiment of the presentinvention;

FIG. 2 is a schematic diagram useful in explaining the operation of thepresent invention;

FIG. 3 is a schematic diagram further illustrating the operation of thepresent invention;

F IG. 4 is a graphic illustration showing the derivation of the outputsignal of the present invention;

FIG. 5 is a schematic diagram of another embodiment of the presentinvention;

FIG. 6 is a schematic diagram of still another embodiment of the presentinvention; and

FIG. 7 is a schematic diagram of the light responsive device used in theembodiment shown in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 in detail,a lens device 1 includes an optical lens 3. The lens 3 has first andsecond electrically responsive optical gates 5 and 7 disposed thereon.In the present example, each optical gate may be com prised of liquidcrystal or other material which responds to a first voltage to becomeopaque and to a second voltage to become clear. The first optical gate 5is semicircular in configuration and is shown opaque in the figure.Optical gate 5 receives an exciting voltage through leads 9 and 11 froma control means 13. The second optical gate 7 is also semicircular andreceives exciting voltage from the control means 13 through leads 15 and17. The first and second semicircular light gates 5 and 7 arejoined atthe center line- 19 thus dividing the lens device 1 into two selectivelygated light paths, one blocked and one unblocked each receiving adistinct control signal from the control circuit 13. The first opticalgate 5 at a given instant may for example have a voltage supply whichwould render it opaque while the second optical gate 7 has a potentialapplied thereto rendering it clear, as shown in the figure. The controlmeans 13 is also operative to move the lens device 1 in forward F andreverse R directions and may be implemented to be coordinated with andcontrol the movement of a principal lens 14 of a camera in which casethe movement of the principal lens 14 which focusses an image of anobject on alight sensitive film 16, may be terminated when the focusdetecting system indicates that the image on the light sensitive film I6is in sharpest focus.

A light responsive sensing device 21 includes five photo responsiveelements 23, 25, 27, 29, and 31. The five photo responsive elements,which in the present example are photo-resistive, are series connectedbetween +V and V reference potentials. The common junctions between thephoto resistive elements provide electrical signals A, B, C, and D whichare, in turn, ap-

plied to a signal processing circuit 33. The photo resistive elements23, 25, 27, 29 and 31 are arranged in the light responsive sensingdevice 21 to receive light from a relatively distant object, (not shown)which passes through the unblocked portion of the lens device 1.

Each of the electrical signals A, B, C and D is applied to an associatedamplifier 35, 37, 39 and 4], respec tively, through respectivecapacitors 34, 36, 38, and 40. The output from each amplifier 35, 37,'39 and 41 is connected through the anode to cathode path of associateddiodes 51, 53, 55 and 57, respectively, to one terminal of theassociated capacitor 43, 45, 47 and 49, respectively. The otherterminals of the capacitors 43, 45, 47v and 49 are connected to a commonreference potential of, for example, zero volts. The cathode terminalsof the diodes 51, 53, 55 and 57 are also connected through associatedresistors 59,, 61, 63 and 65 to a common point 67. The common point 67is connectcd to a first input terminal of an amplifier 69. A secondinput terminal of the amplifier 69 is connected to the common referencepotential. The output terminal of the amplifier 69 is connected througha feedback resistor 71 to the first input terminal thereof. 'The outputof the amplifier 69 also provides an output signal from the signalprocessing circuit 33 to a valley detector circuit 73. The valleydetector circuit, in turn, provides a signal which is applied to thecontrol means 13 to control the movement of the lens device 1 andconcurrently to control the movement of a principal focussing lens 14 ofa camera.

It should be noted that the light responsive sensing device 21 includingthe photo resistive elements 23, 25, 27, 29 and 31 is only shownschematically in FIG. 1 and that in an actual construction the faces ofthe photo resistive elements 23, 25, 27, 29 and 31 are actually facingthe lens device 1 to respond to the light passing therethrough.

In explaining the operation of the present invention, reference is nowhad to FIGS. 2, 3 and 4. In FIG. 2, an object 75, the image of which isto be focussed, includes 2 points of reference P1 and P2. Each point ofreference has two radiation beams emanating therefrom. Beams 77 and 79emanate from point P1 and beams 81 and 83 emanate from point P2. In FIG.2, radiation beams from the points on the object 75 pass through theupper portion of the lens device 1 since the upper mask is clear and thelower mask 7 is opaque. The light responsive sensing device 21 isarranged to receive that radiation and provide the resultant signals A,B, C and D as shown in FIG. 1. In FIG. 2, the lens device 1 is shown ina position at which the image formed on the light responsive sensingdevice 21 is in focus. It should be noted, that the radiation beams 77and 79 emanating from the point P1 form a single point on the photoresponsive element 31 of the light responsive sensing device 21.Similarly, the beams 81 and 83 from the point P2 of the object 75converge to form an image of the point P2 on the photo resistive element23 of the light responsive sensing device 21.

FIG. 3 shows the optical system of FIG. 2 at a time when the lower mask7 of the lens device 1 is transparent. thereby allowing light to passtherethrough, and the upper semicircular mask 5 is opaque therebyblocking radiation from passing thercthrough. Again, in. FIG.

3, the lens device 1 is in the position at which an image of the imageis focussed upon the light responsive device 21. The radiation beams 85and 87 converge to form an image of the point P1 upon the photoresistive element 31, while radiation beams 89 and 91 converge to forman image of the point P2 upon the photo resistive element 23. It cantherefore be seen from FIGS. 2 and 3 that when the lens device 1 is inthe proper focus position, the alternate blocking of a portion of theoptical path between the object 75 and the light responsive sensingdevice 21 has no substantial effect on the resultant signals A, B, C andD provided by the light responsive sensing device 21. That is, as thelens device 1 is switched between top-half transmission and bottomhalftransmission, substantially the same amount of illumination will fall oneach photo resistive element 23, 25, 27 and 29 and 31 of the lightresponsive sensing device 21. The electrical signals A, B, C and D willnot vary substantially i.e. they will vary minimally and appear as DC.signals when the lens device 1 is in the proper focus position. However,if the lens device 1 were moved forward or rearward of its focusposition for any given object, there will be a substantial variation inthe electrical signals A, B, C and D as the top and bottom portions ofthe lens device 1 are alternately rendered transmissive.,For example, inFIG. 2 if the lens device 1 were moved away from its focus positiontoward the light responsive sensing device 21, the light responsivesensing device 21 would appear at a plane as shown at 93 in FIG. 2 withrespect to the lens device 1. On the other hand, if the lens device 1were moved out of its focus position toward the object 75, the lightresponsive sensing device 21 would appear at a plane in the position 95with respect to the lens device 1. In either case, there will be asubstantial change in the magnitude of the electrical signals A, B, C,and D generated as the top and bottom halves of the lens device 21 arealternately rendered transmissive. For example, with the lens device 1moved toward the light responsive device 21 so the light responsivedevice 21 appears at the .position 93 with respect to the lens device 1,radiation beams from either point P1 or P2 on the object 75 will notfall on the same photo resistive element as the top and bottom portionsof the lens device are alternately rendered transmissive. Radiation fromthe point P1, when the top portion of the lens device 1 is transmissive,would illuminate the photo resistive element 27 and part of the photoresistive element 29. However, when the bottom portion of the lensdevice 1 is transmissive, radiation from the point P1 would illuminatethe photo resistive element 31 and a portion of the photo resistiveelement 29. Therefore, as different por- 'tions of the optical pathbetween the object 75 and the light responsive device 21 are alternatelyblocked out, the effective illumination distribution falling on thelight responsive device 21 will vary and as a result the electricalsignals A, B, C and D will also vary substantially and appear as A.C.signals. The signals thus provided are generally shown in FIG. 4.

In FIG.. 4 the waveform represents a typical one of the signals A, B, Cor D. Substantial changes in voltage are shown as the top and bottomportions of the lens device are alternately rendered conductive and thelens device 1 is not in its proper focus position. However, as the lensdevice l approaches bestfocus position, those changes in each electricalsignal are substantially reduced and approach a minimal change when thelens device 1 is actually in its best focus position. Each of the ACelectrical signals A, B, C and D is amplified, rectified, and filteredto provide a filtered signal which corresponds to the envelope 97 of thepeak value. Those envelope or filtered signals are then added togetherby a signal processing circuit 33 to provide the output signal which isapplied to the valley detector circuit 73. The valley detector circuit73 detects the valley or minimum value of the output signal, at whichtime a signal is provided to the control means 13 to terminate themovement of the lens device 1 and the principal lens 14 in its detectedbest focus position.

FIG. 5 shows another embodiment of the present invention. A lens device101 is movable in forward F and reverse R directions to vary thecondition of focus of an image formed thereby on a light responsivesensing device 103. In the optical path between the lens device 101 andthe light responsive sensing device 103, a light gate 105 is interposed.The light gate 105 is comprised of two apertured or slitted masks, amovable mask 107 and a stationary mask 109. Radiation from an object,the image of which is to be focussed on the light responsive device 103,is selectively passed through the apertures in the gate 105 to form animage of that object on the light responsive sensing device 103. Themovable mask 107 of the gate 105 moves up and down with an oscillatorymovement to be defined more particularly hereinafter, to effectivelyselect the radiation beams impinging thereon at various angles from theobject (not shown). When the moveable mask 107 of the light gate 105 isin one extreme position, only light rays passing through the lens device101 at a first angle with respect to the central axis of the lens device101 will impinge upon the light responsive sensing device 103. As themoving portion 107 of the light gate 105 moves towards its other extremeposition, radiation having different angles ofincidencc will be allowedto fall on the light responsive sensing device 103 until at the otherextreme position only radiation from a second extreme angle will passthrough the gate 105. Because of the geometry of the system, when themovable mask is in one extreme position, the angle of light passageeffectively allows only light passing through the top portion of thelens 101 to impinge on the light responsive sensing device 103, and whenthe movable mask is in the other extreme position, only radiationpassing through the bottom portion of the lens 101 is passed to form animage on the light responsive sensing device 103.

The light responsive device 103 is comprised in the illustrativeembodiment, of five photo resistive elements serially connected between+V and V potentials. As was the case in FIG. 1, the electrical signalsA, B, C and D provided by the light responsive device 103 are applied toa signal processor circuit 111. The output from the signal processor issupplied to a valley detector circuit 113 which. in turn, provides asignal which is applied to a control device 115 which controls themovement of the lens device 101 and an objective is moved between itsextreme positions. That will be true since when the image is focussed onthe light responsive sensing device 103, the light distribution of thatimage will be the same as the mask 107 moves since the image will remainfocussed regardless of which particular light rays effect the formationof that image. On the other hand, when the lens device 101 is not in itsproper focus position, the image formed on the light responsive sensingdevice 103 will not be in the best condition of focus and the lightdistribution pattern appearing on the light responsive device 103 willvary as the movable mask 107 of the light gate 105 moves between itsextreme positions. When the lens device 101 is not in its proper focusposition, the radiation intensity distribution appearing on the lightresponsive device 103 will depend upon which rays of light are passedfrom the object from the lens device 101 and the light gate 105.Therefore, as the movable portion 107 oscillates between its extremepositions while the lens device 101 is concurrently being progressivelymoved in either the forward F or rearward R directions, the electricalsignals A, B, C and D will exhibit a degree of change and appear as A.C.signals when the lens device 101 is not in its focus position;substantially no change will be exhibited, however, and signals A, B, Cand D will appear as DC. signals when the lens device 101 is in itsproper focus position. The speed of oscillation of movable mask 107 ismany times greater than the speed with which the lens device 101 movesin forward and rearward directions and, consequently, for any givenposition of the lens device 101, the movable mask 107 completes at leastone complete scan between its extreme positions. Therefore, by sensingwhen the electrical signals A, B, C and D exhibit a minimum change, thelens device 101 is in its best focus position. The signal processorcircuit 111 of FIG. 5 may be identical to the signal processor circuit33 of FIG. 1. The valley detector circuit 113 is substantially the sameas the valley detector circuit 73 in FIG. 1 and provides a signal to thecontrol means 115 when a valley or minimal variation in the signal fromthe processor 111 is detected. That minimal or valley point on theoutput signal of the signal processor circuit 111 is indicative of thefact that the lens device 101 is in its proper focus position.Accordingly, the control means 115 may be designed to terminate themovement of the lens device 101, the movable mask 107 and the objectivelens 102 when that minimal or valley value of the output signal has beendetected.

In FIG. 6 a lens device 117 includes an optical lens 119 and a rotating,semi-circular. opaque mask 121 mounted thereon. The lens device 117, ascontrolled by a control means 123, is movable in forward F and reverseor rearward R directions. The control means 123 controls the movement ofan objective lens 118 which is operable to focus an image of an objecton a light sensitive film 120. The control circuit 123 also controls theoperation of a motor 125 which in turn effects the rotation of thesemi-circular mask 121. The semicircular mask 121 is made to rotate at afaster rate than the linear cyclic movement of the lens device 117 so aplurality of rotations of the semi-circular mask 121 may occur forsubstantially any given incremental movement of the lens device 117 inthe forward F or rearward R directions.

A photo responsive device 127 is comprised. in the illustrativeembodiment, of 16 photo-voltaic elements arranged in a square patternwith fourrows of four elements each. Each photo-voltaic element providesits own electricalsignal as is'more clearly-shown in FIG. 7. Theelectrical signal provided by each of the photovoltaic elements in thelight responsive device 127 is representative of the intensity of thelight falling thereon. The electrical signals thereby provided areapplied to-a processing circuit 129. The electrical signal provided byeach of the photo-voltaic elements of the light responsive device 127 isapplied, through separate serial capacitor-resistor circuits, to acommon summing junction 128 within the processing circuit 129. Thesumming junction 128 is connected to a first input terminal of anamplifier 131, the second input terminal of which is connected to areference potential of, for example, zero volts. A feedback resistor 133connects the output terminal of the amplifier 131 with its first inputterminal. The output terminal of the amplifier 131 provides the outputsignal from the processing circuit 129 which is applied to a valleydetector circuit 135. The valley detector 135, in turn, provides asignal which is applied to the control means 123 for controlling themovement of the lens device 117 and the motor 125.

To better understand the operation in the embodiment shown in FIG. 6, itis helpful to refer back to the operation of the embodiment shown inFIG. 1. In FIG. 1, when the image was properly focussed on the lightresponsive sensing device 21, substantially no changes would occur' inthe signals A, B, C and D as opposite halves of the lens device 1 werealternately rendered transmissive. That was truesince only one-half ofan optical lens is sufficient to focus an image of an object. In likemanner, with the rotating blocking mask 121 in any given position suchas, for example, blocking the bottom half of the lens 119 as shown inFIG. 6, an image of the object will be formed on a light responsivedevice 127 from radiation comming from the object and passing throughthe top portion of the lens 119. If the image formed on the lightresponsive sensing device 127 is properly focussed, the lightdistribution pattern ofthat image will not be changed as the mask 121 isrotated about the. axis of the lens 119 since the image will remain infocus regardlessof which particular radiation beams formit. However, ashereinbefore explained in connection with FIGS. 2 and 3, when the imageappearing at the light responsive sensing device 127 is not properlyfocussed,'the selection of the radiation beams from the object whichform the image will have a substantial effect on the radiationdistribution on the light responsive sensing device. That is, as therotating disc 121 rotates about the axis of the lens 119, it effectivelyselects different radiation beams by selecting different paths throughwhich radiation from the object will pass in forming an image on thelight responsive sensing device 127. The rotating disc 121 thereforewill also effect a variation in the radiation intensity distributionwhen the image formed on the light responsive sensing device 127 is notproperly focussed Therefore, when the image on the light responsivesensing device 127 is properly focussed, no significant changes willappear in the signals provided by the photo-voltaic elements in thelight responsive device 127 as the rotating mask 121 rotates about theaxis of the lens 119. Since no changes in the signals from thephoto-voltaic elements occur, no A.C. signals will be coupled throughthe coupling portion of the signal processing circuit 129 and the outputsignal from the signal processing circuit will be at a minimal value.That minimal value is detected by the valley detector circuit which, inturn, ap-

- plies a signal to the control circuit 123 which will terminate themovement of the lens 119 in the proper focus position. If the image onthe light responsive device 127 is not properlyfocussed, the lightdistribution pattern will change as the rotating mask 121 rotates aboutthe lens 119. The change in the distribution pattern will accordinglyeffect the generation of AC signals by the photo-voltaie elements in thelight responsive device 127. Those AC signals are coupled through theirassociated capacitor and resistor coupling circuits in the signalprocessing circuit 129 and are added together at the common or summingjunction 128. When those AC signals are summed together and amplified bythe amplifier 131, an output signal will be provided by the signalprocessing circuit 129 which will be significantly greater in magnitudethan the minimal signal provided when substantially no variation in thelight distribution pattern of the image on the light responsive sensingde vice 127 appeared. Since the output signal when the image is notproperly focussed is not at a minimal value, the valley detector circuitwill not operate to provide a signal to terminate the movement of thelens device 117 and therefore the lens device 117 will continue to bemoved in either the forward or rearward direction until such a minimallyvalued output signal from the signal processing circuit appears.

Thus, there has been provided, in accordance with the present invention,an improved focus detecting apparatus which is more accurate andreliable than prior v art devices and which is simple in design andrequires neither frequency component analysis nor matching of lightresponsive elements.

The embodiments of the present invention in which an exclusive propertyor privilege is claimed are defined as follows:

1. A focus condition detecting apparatus for use in focussing an imageof an object, comprising light sensing means including a plurality oflight sensing elements spatially distributed in an array in a givenplane, each of said elements providing an electrical signalrepresentative of the intensity of light falling thereon,

optical means having a central axis and an entrance pupil and positionedto form on said plane, over an optical path through said pupil, a lightimage of said object,

cyclically operated light gating means comprising first and secondopaque masks positioned substantially at said optical means andextending transversely of said axis and spaced from each othertherealong, and arranged to have cyclic reciprocating relative motion inthe direction of their length.

each of said masks including a plurality of slit apertures along itslength so that said cyclic motion causes said gating means to cyclicallyblock predetermined spatially separate areas of said entrance pupil ofsaid optical means, to cause light from said object to reach saidsensing means cyclically from said spatially separate areas and to causesaid image and hence the distribution of light to shift cyclically oversaid array of sensing elements to an extent dependent upon the extent towhich said image is out of focus on said plane, said image and lightdistribution remaining substantially stationary on said array of sensingelements, notwithstanding 9 said cyclic operation of said gating means,when said image is in focus on said plane, and means connected to saidsensing elements and responsive to said signals to provide an outputsignal representative of the condition of focus of said image on saidplane. 2. The focus detecting apparatus as set forth in claim 1 whereinsaid means responsive to said electrical signals includes means forrectifying each of said electri cal signals to provide a rectifiedsignal for each of said electrical signals, and means responsive to saidrectified signals for summing said rectified signals and providing anoutput signal, the magnitude of which is related to the condition offocus of said image.

3. The focus detecting apparatus as set forth in claim 2 wherein saidmeans responsive to said rectified signals includes means for filteringsaid rectified signals. 4. The focus detecting apparatus as set forth inclaim 1 wherein said elements are photo resistors.

5. The focus detecting apparatus as set forth in claim 1 wherein saidelements are photo-voltaic cells.

6. An automatic focussing apparatus for use in focussing a principleimage of an object, comprising lens means for forming a principle lightimage of an object on a light sensitive film, light sensing meansincluding a plurality of light sensing elements spatially distributed inan array in a given plane, each of said elements providing an electricalsignal representative of the intensity of light falling thereon, opticalmeans having a central axis and an entrance pupil and positioned to formon said plane, over an optical path through said pupil. an auxiliarylight image of said object, cyclically operated light gating meanscomprising first and second opaque masks positioned substantially atsaid optical means extending transversely of said axis and spaced fromeach other therealong, and arranged to have cyclic reciprocatingrelative motion in the direction of their length, each of said masksincluding a plurality of slit apertures along its length so that saidcyclic motion causes said gating means to cyclically block predeterminedspatially separate areas of said entrance pupil of said optical means,to cause light from said object to reach said sensing means cyclicallyfrom said spatially separate areas and to cause said auxiliary image andhence the distribution oflight to shift cyclically over said array ofsensing elements to an extent dependent upon the extent to which saidauxiliary image is out of focus on said plane, said auxiliary image andlight distribution remaining substantially stationary on said array ofsensing elements, notwithstanding said cyclic operation of said gatingmeans, when said auxiliary image is in focus on said plane,

means connected to said sensing elements and responsive to said signalsto provide an output signal representative of the extent of said cyclicshifting of said auxiliary image on said plane, and

control means connected to receive said output signal and connected toadjust the relative positions of said optical means and said plane, andsimultaneously to adjust the relative positions of said lens means andsaid film,

said control means being responsive to said output signal to adjust therelative positions of said optical means and said plane to those atwhich said output signal indicates a minimum in said cyclic shifting ofsaid auxiliary image on said plane, the correspond ing adjusted relativepositions of said lens means and said film then being those which causesaid principle image to be focussed on said film.

7. The focus detecting apparatus as set forth in claim 1, wherein saidfirst mask is stationary with respect to said optical means,

said second mask is arranged for cyclic reciprocating movement in thedirection ofits length with respect to said first mask to impart saidcyclic relative motion between said masks, and

said masks are so positioned with respect to said optical means thatthey cyclically effectively mask the two halves of said optical meansalternately.

1. A focus condition detecting apparatus for use in focussing an imageof an object, comprising light sensing means including a plurality oflight sensing elements spatially distributed in an array in a givenplane, each of said elements providing an electrical signalrepresentative of the intensity of light falling thereon, optical meanshaving a central axis and an entrance pupil and positioned to form onsaid plane, over an optical path through said pupil, a light image ofsaid object, cyclically operated light gating means comprising first andsecond opaque masks positioned substantially at said optical means andextending transversely of said axis and spaced from each othertherealong, and arranged to have cyclic reciprocating relative motion inthe direction of their length, each of said masks including a pluralityof slit apertures along its length so that said cyclic motion causessaid gating means to cyclically block predetermined spatially separateareas of said entrance pupil of said optical means, to cause light fromsaid object to reach said sensing means cyclically from said spatiallyseparate areas and to cause said image and hence the distribution oflight to shift cyclically over said array of sensing elements to anextent dependent upon the extent to which said image is out of focus onsaid plane, said image and light distribution remaining substantiallystationary on said array of sensing elements, notwithstanding saidcyclic operation of said gating means, when said image is in focus onsaid plane, and means connected to said sensing elements and responsiveto said signals to provide an output signal representative of thecondition of focus of said image on said plane.
 2. The focus detectingapparatus as set forth in claim 1 wherein said means responsive to saidelectrical signals includes means for rectifying each of said electricalsignals to provide a rectified signal for each of said electricalsignals, and means responsive to said rectified signals for summing saidrectified signals and providing an output signal, the magnitude of whichis related to the condition of focus of said image.
 3. The focusdetecting apparatus as set forth in claim 2 wherein said meansresponsive to said rectified signals includes means for filtering saidrectified signals.
 4. The focus detecting apparatus as set forth inclaim 1 wherein said elements are photo resistors.
 5. The focusdetecting apparatus as set forth in claim 1 wherein said elements arephoto-voltaic cells.
 6. An automatic focussing apparatus for use infocussing a principle image of an object, comprising lens means forforming a principle light image of an object on a light sensitive film,light sensing means including a plurality of light sensing elementsspatially distributed in an array in a given plane, each of saidelements providing an electrical signal representative of the intensityof light falling thereon, optical means having a central axis and anentrance pupil and positioned to form on said plane, over an opticalpath through said pupil, an auxiliary light image of said object,cyclically operated light gating means comprising first and secondopaque masks positioned substantially at said optical means extendingtransversely of said axis and spaced from each other therealong, andarranged to have cyclic reciprocating relative motion in the directionof their length, each of said masks including a plurality of slitapertures along its length so that said cyclic motion causes said gatingmeans to cyclically block predetermined spatially separate areas of saidentrance pupil of said optical means, to cause light from said object toreach said sensIng means cyclically from said spatially separate areasand to cause said auxiliary image and hence the distribution of light toshift cyclically over said array of sensing elements to an extentdependent upon the extent to which said auxiliary image is out of focuson said plane, said auxiliary image and light distribution remainingsubstantially stationary on said array of sensing elements,notwithstanding said cyclic operation of said gating means, when saidauxiliary image is in focus on said plane, means connected to saidsensing elements and responsive to said signals to provide an outputsignal representative of the extent of said cyclic shifting of saidauxiliary image on said plane, and control means connected to receivesaid output signal and connected to adjust the relative positions ofsaid optical means and said plane, and simultaneously to adjust therelative positions of said lens means and said film, said control meansbeing responsive to said output signal to adjust the relative positionsof said optical means and said plane to those at which said outputsignal indicates a minimum in said cyclic shifting of said auxiliaryimage on said plane, the corresponding adjusted relative positions ofsaid lens means and said film then being those which cause saidprinciple image to be focussed on said film.
 7. The focus detectingapparatus as set forth in claim 1, wherein said first mask is stationarywith respect to said optical means, said second mask is arranged forcyclic reciprocating movement in the direction of its length withrespect to said first mask to impart said cyclic relative motion betweensaid masks, and said masks are so positioned with respect to saidoptical means that they cyclically effectively mask the two halves ofsaid optical means alternately.